The present invention relates to an improved method and apparatus for reducing unwanted noise in the chroma component of a quadrature modulated color television picture signal. More particularly, the present invention achieves chroma noise reduction by ordinarily averaging chroma by use of a recirculating delay line which feeds back a predetermined fractional component of the chroma to the input; when vertical transitions are detected, the recirculated chroma component is switched out, in proportion to the magnitude of the luminance or chrominance transition, whichever is greater.
The problem of noise in chroma components of a quadrature modulated color television signal has been previously addressed by workers in the art.
One solution was proposed by Ray Milton Dolby in U.S. Pat. No. 3,978,409, wherein he suggested a boost of chroma only during low level chroma transition conditions. The boost was provided by adding a low level, dynamically modified chroma component to a main chroma path at an encoder stage of a system. A complimentary bucking of the chroma path was provided at a subsequent decoder stage by subtraction of a second dynamically modified chroma component which closely complemented the first dynamic component. The resultant chroma closely modelled the original, with noise reduced during low signal activity periods. When high chroma activity was present, the Dolby system stopped acting, and it left noise components untouched. In fact, the Dolby approach only operated when chroma transitions did not exceed 10% of the maximum chroma amplitude. The practical drawback of the Dolby system insofar as applied to chroma noise reduction is that the human eye is too insensitive to low level chroma transitions, and little is gained in subjective picture quality by limiting chroma noise reduction processing to the low activity chroma elements.
Another prior art approach was disclosed in the Kaiser et al. U.S. Pat. No. 4,064,530 as improved by the approach set forth in the later Rossi et al. U.S. Pat. No. 4,107,739. In that approach, a switching signal provided for soft switching between incoming video and recirculating video on a frame by frame comparison basis. Although Rossi et al. properly identified the need to make the switching waveform wider than the signal to be switched, i.e. starting before and stopping after such signal, a significant drawback of the Rossi et al. approach was that the recirculating video itself provided the means by which the switching control signal was generated. The practical consequence of that approach was that the motion evaluation means was not able to separate true picture motion from noise and other random disturbances occurring over the relatively long period separating like positioned picture elements between each frame. Another significant drawback of the Rossi et al. approach was the high cost of one-frame delay lines or elements, as compared with the one-line delay elements used to implement the present invention.
The present applicant proposed a solution to chroma noise reduction in his U.S. Pat. No. 4,240,105. Therein, this applicant described a comb filter separator for chrominance which effectively reduced noise and unwanted picture artifacts, except at major chrominance transitions. It has been statistically noticed that, generally, a chroma transition coincides with a luminance transition. A luminance transition is usually a very stable signal, with a high signal-to-noise ratio. Consequently, it may be treated as a reliable indicator of the presence of a chroma transition, except in very rare circumstances. Thus, the present applicant proposed to substitute bandpass filtered chrominance in lieu of combed chrominance at the instance of detected vertical luminance transitions, in order to remove unwanted artifacts generated by a comb filter at such transitions. The overall results of the applicant's prior invention were very acceptable, yet that system was complicated by the need for multiple delay lines and was generally more complex.
Others have proposed to reduce chroma noise by averaging chroma over several lines, e.g. four lines. The known approach to implement such averaging was to employ multiple delay lines and combine in phase predetermined amplitude components of the multiple-delayed chroma. That approach had the drawback of complexity and expense in requiring multiple delay lines and summing junctions.
While a recirculating delay line approach is suggested by Dolby in his patent described above, and is known in the prior art, there are drawbacks in picture quality which present themselves as smearing at vertical transitions in the picture. The present invention advantageously combines the inexpensiveness of a recirculating delay line with the linearly proportional or "soft" switch to provide unrecirculated chroma only, during vertical subject matter transitions in the picture to achieve a truly superior color picture in which the chroma noise has been effectively reduced.